A comparator type DC-DC converter is known as a power regulator. For example, a so-called step-down DC-DC converter, which generates an output voltage that is lower than its input voltage, includes a switch circuit, which is supplied with the input voltage, a coil, which is coupled to the switch circuit, and a smoothing capacitor, which is coupled between the coil and an output terminal. Current flows to the coil via the switch circuit when the input voltage is supplied. The smoothing capacitor smoothes the coil current to generate output voltage. The output voltage includes ripple voltage (ripple component) caused by the coil current and the equivalent series resistance (ESR) of the smoothing capacitor. Hence, a comparator compares the output voltage with a constant base voltage and, for example, activates the switch circuit when the output voltage becomes lower than the base voltage. Then, after a certain time elapses, the switch circuit is inactivated to control the output voltage of the DC-DC converter.
As described above, in a DC-DC converter that switches a switch circuit to generate output voltage, stabilized output voltage is favorable, that is, the output voltage having less ripple components is favorable. To meet this requirement, a DC-DC converter using a smoothing capacitor having a small equivalent series resistance has been discussed (for example, refer to US 2005/0286269). In such a DC-DC converter, the base voltage input to the comparator to control switching includes a ramp voltage portion, which is ramped at a given inclination.
The switch circuit functions under a switching duty that, for example, varies in accordance with changes in the load. For instance, when the load decreases, the inactive period is lengthened to lower the output current. Since the inclination of the ramp voltage portion in the base voltage is fixed, the voltage becomes high at the point of time in which the output voltage fed back to the comparator intersects the base voltage. This stabilizes the output voltage at a voltage that is higher than the desired target voltage. When the input voltage decreases, the output voltage stabilizes at a voltage that is lower than the desired target voltage. In this manner, the conventional DC-DC comparator is not applicable to a duty that varies in accordance with changes in the input voltage. This may have an adverse effect on line regulation.